Aromatic polyimides, which show heat resistance at the level highest among polymers and are also superior in mechanical and electrical properties, have been used as raw materials in a wide range of applications.
On the other hand, aromatic polyimides are generally poor in processability and not suitable for melt molding or for use as a matrix resin for fiber-reinforced composite materials. For that reason, imide oligomers modified with a thermally crosslinkable group at the terminals were proposed. In particular, imide oligomers modified with 4-(2-phenylethynyl)phthalic anhydride at the terminals are seemingly superior in the balance of moldability, heat resistance, and mechanical properties, and such imide oligomers are described, for example, in Patent Documents 1, 2, and 3 and Nonpatent Literatures 1 and 2. For the purpose of providing a terminally modified imide oligomer giving a cured product superior in heat resistance and mechanical properties and thus superior in usefulness and the cured product thereof, the Patent Document 1 discloses a terminally modified imide oligomer having a logarithmic viscosity of 0.05 to 1 prepared in reaction of 2,3,3′,4-biphenyltetracarboxylic dianhydride having a bent and non-planar structure, an aromatic diamine compound, and 4-(2-phenylethynyl)phthalic anhydride and a cured product thereof. It also describes that it is possible, as its advantageous effects of the invention, to obtain a new terminally modified imide oligomer higher in usefulness and also to obtain a cured product of the new terminally modified polyimide superior in heat resistance and mechanical properties such as modulus, tensile strength, and elongation.
However, these terminally modified imide oligomers are only soluble at a concentration of 20 wt % or less in organic solvents such as N-methyl-2-pyrrolidone (hereinafter, referred to simply as NMP) at room temperature (the room temperature means a temperature of 23° C.±2° C. in the present description), and there is observed a phenomenon that the varnish thereof gels within several days, when left still, and thus, such imide oligomers had a problem that it was difficult to store the varnish at higher concentration, as stabilized for an extended period of time.
As described for example in Patent Documents 3 and 4, a method of copolymerizing an acid anhydride monomer having a further bent and non-planar structure, such as 2,2,3′,3′-biphenyltetracarboxylic dianhydride, is used as a means of raising the solubility, but it generally leads to decrease in breaking elongation of the resulting polymer, making it brittler.
As described in Patent Documents 4 and 5, a method of copolymerizing a diamine monomer having a sterically bulky structure such as 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene can also be used, but it also generally leads to decrease of the glass transition temperature and the breaking elongation of the resulting polymer, making it brittler.
As described in Patent Document 5, for example in the case of an imide oligomer for injection or transfer molding, the polymerization degree is lowered or the structure of the diamine used is made more flexible, to make the imide oligomer more flowable, but such a treatment only leads to significant drop of the glass transition temperature of the terminal cured product, although it makes the imide oligomer more soluble.